LIGHTNING 355

and then quickly return, and so on through an endless variety of places and conditions.

Doubtless many reported cases of ball lightning, probably the great majority, are entirely spurious, being either fixed or wandering brush discharges or else nothing other than optical illusions, due, presumably,

But here, too, as in the case of rocket lightning,

tfie amolmfanH excellence of observational evidence forbid the assumption that all such phenomena are merely subjective. Possibly, in some instances, especially those in which it is seen to fall from the clouds, ball lightning may be only extreme cases of rocket lightning, cases in which the discharge, for a time, just sustains itself. A closely similar idea has been developed in detail by Toepler.1 It might either disappear wholly and noiselessly, as often reported, or it could, perhaps, suddenly gain in strength and instantly disappear as sometimes observed, with a sharp, abrupt clap of thunder.

To say that all genuine cases of ball lightning, those that are neither brush discharges nor mere optical illusions, are stalled thunderbolts, certainly may sound very strange. But that, indeed, is just what they are, according to the above speculation, a speculation that recognizes no difference in kind between streak, rocket, and ball lightning; only differences in the amounts of ionization, quantities of available electricity and steepness of potential gradients.

Sheet Lightning. — When a distant thundercloud is observed at night, one is quite certain to see in it beautiful illuminations, appearing like great sheets of flame, that usually wander, flicker and glow in exactly the same manner as does streak lightning, often for well nigh a whole second, and,....0£casionall3i^^^ In the daytime and in full

sunlight, the phenomenon, when seen at all, appears like a sudden sheen that travels jmd spreads, here and there, over the surface of the cloud. Certainly in most casesTsoTar" as definitely known in all cases, this is only reflection from the body of the cloud of streak lightning in other and invisible portions. Often a blurred, yellowish streak is seen through the thinner portions of the intervening cloud. Occasionally, too, the cloud is wholly cleared in places where, of course, the discharge, usually, is white and dazzling. Conceivably a brush, or coronal, discharge may take place from the upper surface of a thunderstorm cloud, but one would expect this to be either a faint continuous glow, or else a momentary flush coincident with a discharge from the lower portion of the cloud to earth, or to some other cloud. But, as already stated, only reflection is definitely known to be the cause of sheet lightning. Coronal effects seem occasionally possible, but that they ever are the cause of the phenomenon in question has never clearly been established and appears very doubtful. It has often been asserted, too, that there is a radical differ-

i /Irm Phii*.. 2: 623. 1900.